This invention relates to a detecting device for detecting excessive voltage drop of a battery or cell, in which a predetermined warning voltage is detected in the gradual voltage drop of the battery. For small electronic instruments such as electronic watches, batteries or cells are generally in use as the power source. The battery or cell keeps initially an approximately constant voltage V.sub.B for some long time, as shown in FIG. 1. The subsequent discharge of the battery causes a gradual voltage drop of the battery output. An electronic device generally operates in an unstable manner or stops entirely when it is supplied, for its operation, with a voltage less than a voltage V.sub.1 from the power source or battery. For assuring stable operation of the electronic device, it is necessary to detect the battery voltage drop greater than a predetermined voltage and to exchange the battery with a new one before unstable operation begins in the device.
Conventional excessive voltage drop detectors proposed for a battery or cell detect voltage drop in a voltage dividing circuit which consists of a plurality of resistors or of a combination of resistors and condensers. But, it is very difficult to choose resistors or condensers of equal value for the voltage dividing circuit, and, therefore, detecting accuracy of the voltage as the dividing accuracy is not so high. These voltage dividing circuits are difficult to manufacture in such compact integrated circuits as are used in small electronic devices such as electronic watches.